The present invention relates to a negative yarn feeder with incorporated position detector.

As known, the so-called "negative" yarn feeders for textile machines generally comprise a stationary drum on which a swivel arm operated by a motor driven by a control unit winds a plurality of yarn loops forming a weft reserve. Upon request from the loom, the loops are unwound from the drum, then pass through a braking device which controls the tension of the yarn, and finally are fed to the loom.

In order to control the angular position and the speed of the winding arm, the known yarn feeders incorporate a position detector which comprises a fly-wheel keyed to the driving shaft and bearing a plurality of equally-spaced, equally-polarized magnets on its periphery, as well as a reference magnet with opposite polarity. A pair of stationary Hall sensors are arranged near the fly-wheel, one sensor being sensitive to a polarity, the other being sensitive to the opposite polarity. This detector allows a position information and a speed information of the winding drum to be measured and to be used as feedback signals for the control unit.

The above known yarn feeders have the drawback that the winding drum can be positioned easily and accurately only at a limited number of angular positions, each of which corresponds to a magnet on the fly-wheel, with a resolution which consequently depends on the number, as well as on the position, of the magnets.

On the other hand, the number of magnets positionable on the fly-wheel is conditioned by both manufacturing and functional limitations, because the magnets must be sufficiently spaced from one another for the Hall sensors to be capable of distinguishing their signals. Moreover, since the magnets must necessarily be made of a material having a very high value of residual magnetic induction, such as Neodimium, Iron-Boron, and the like, having a large number of magnets results in a considerable increase of the cost of the detector.

Another drawback of the known yarn feeders, which derives from the use of position detectors of the above-described type, is that the speed signal calculated by measuring the time intervals between subsequent magnet pulses, has a frequency band which is intrinsecally correlated to the speed of rotation of the motor. Particularly, the lower the speed of rotation of the motor, the larger the time interval between subsequent pulses form the Hall sensors and, consequently, the lower the frequency band of the feedback signal. This circumstance leads to limit the frequency band of the speed control loop of the control unit, thereby affecting the band and the stability thereof.

A further drawback of the known yarn feeders is that, in order to univocally measure the position of the winding drum, it is always required to make the reference magnet pass in front of the Hall sensor (so-called "zero-search" procedure), with consequent lengthening of the time required for positioning the arm.

Therefore, it is a main object of the present invention to provide a negative yarn feeder incorporating a position detector which generates a continuously variable position signal and which allows to calculate a speed signal having a frequency band that is constant with respect to the speed of rotation.

It is another object of the invention to provide a weft feeder having a position detector which allows to measure the position of the winding drum immediately and univocally, without requiring zero-search procedures.

The above objects and other advantages, which will better appear below, are achieved by a negative yarn feeder with incorporated position detector having the features recited in claim 1, while the dependent claims state other advantageous, though secondary features of the invention.

The invention will be now described in more detail with reference to a preferred, non-exclusive embodiment, shown by way of non limiting example in the attached drawings, wherein:

• Fig. 1 is a view in side elevation of a yarn feeder with incorporated position detector according to the invention;
• Fig. 2 is a perspective view showing the position detector of Fig. 1 isolately;
• Fig. 3 is a diagram showing the voltage signal generated by the position sensor of Fig. 2 as a function of the angular position.

With initial reference to Figs. 1, 2, a yarn feeder 10 for textile machines comprises a stationary drum 12 on which a motorized swivel arm 14 winds a plurality of yarn loops forming a weft reserve RT. Swivel arm 14 is operated by a motor 15 driven by a control unit UC (which is only diagrammatically shown in Fig. 1). Upon request from the loom, the loops are unwound from drum 12, then pass through a braking device 18 known per se, which is supported on a stationary arm 19 projecting from the motor housing parallel to the axis of the drum. Braking device 18 is adapted to control the tension of yarn F unwinding from the drum.

A position detector 20 is incorporated within the motor housing for controlling the angular position and the speed of the winding arm. According to the invention, position detector 20 comprises an annular permanent magnet 22 which is keyed to a shaft 15a of motor 15 and is diametrally magnetized with a sinusoidal distribution of the magnetization, as well as a pair of Hall sensors 24, 26 which are arranged near permanent magnet 22 in order to detect the diametral component of the magnetic field generated by the magnet, and are angularly spaced at 90° from each other. Hall sensors 24, 26 are mounted on a printed circuit 28 (only diagrammatically shown in Fig. 1) which is connected for sending analog signals to control unit UC via an analog to digital converter A/D. In Fig. 2, lines F1, F2 represent the curve of the intensity of the diametral component of the magnetic field generated by magnet 22 as a function of the angular position along its periphery, whereby the nearer the line to the inner edge of the magnet, the higher the intensity at that angular position (the highest intensity is detected by Hall sensor 24, the lowest intensity is detected by Hall sensor 26).

Fig. 3 is a test diagram showing the curves of the analog voltage values V1, V2 sent by a pair of Hall sensors 24, 26 as a function of the angular position &OHgr; of magnet 22 and, consequently, of the winding arm. Signals V1, V2 are sinusoidal, with an offset of 90° from each other. In the practise, the amplitude amp1 and the offset value ofs1 of the signal from the first Hall sensor 24 are slightly different from the amplitude amp2 and the offset value ofs2 of the signal from the second Hall sensor 26, due to the inevitable manufacturing/installing tolerances. These values can be automatically calculated by means of a self-tuning procedure in which the motor is driven to rotate while the control loop is open, i.e., without speed feedback, and with a fixed frequency of rotation, while the control unit samples the values of the voltage signals V1, V2. After a certain number of cycles, the control unit calculates the maximum value and the minimum value of the signals sent from Hall sensors 24, 26, Max(V1), Min(V1) and Max(V2), Min(V2) respectively, and calculates the amplitude values and the offset values on the basis of the following equations: $$\mathrm{amp}\mathrm{1}\mathrm{=}\frac{\mathrm{Max}\left(\mathrm{V},,\mathrm{1}\right)\mathrm{-}\mathrm{Min}\left(\mathrm{V},,\mathrm{1}\right)}{\mathrm{2}}\mathrm{,}$$ $$\mathrm{ofs}\mathrm{1}\mathrm{=}\mathrm{Min}\left(\mathrm{V},,\mathrm{1}\right)\mathrm{+}\frac{\mathrm{Max}\left(\mathrm{V},,\mathrm{1}\right)\mathrm{-}\mathrm{Min}\left(\mathrm{V},,\mathrm{1}\right)}{\mathrm{2}}\mathrm{,}$$ $$\mathrm{amp}2\mathrm{=}\frac{\mathrm{Max}\left(\mathrm{V},,2\right)\mathrm{-}\mathrm{Min}\left(\mathrm{V},,2\right)}{\mathrm{2}}\mathrm{,}$$ $$\mathrm{ofs}2\mathrm{=}\mathrm{Min}\left(\mathrm{V},,2\right)\mathrm{+}\frac{\mathrm{Max}\left(\mathrm{V},,2\right)\mathrm{-}\mathrm{Min}\left(\mathrm{V},,2\right)}{\mathrm{2}}\mathrm{.}$$

Control unit UC is programmed for calculating the absolute angular position &OHgr; of motor shaft 15a, and consequently of winding arm 14, in real time, on the basis of the signals sent from Hall sensors and of the amplitude values and offset values calculated as above. In particular, such absolute angular position &OHgr; is univocally defined from the equation: $$\mathrm{\&OHgr;}\mathrm{=}\arcsin \left(\frac{\mathrm{V}\mathrm{1}\mathrm{-}\mathrm{ofs}\mathrm{1}}{\mathrm{amp}\mathrm{1}}\right)\mathrm{+}\mathrm{sign}\left(\mathrm{V},,\mathrm{1},\mathrm{-},\mathrm{ofs},,\mathrm{1}\right)\mathrm{\times }\mathrm{isneg}\left(\mathrm{V},,\mathrm{2},\mathrm{-},\mathrm{ofs},,\mathrm{2}\right)\mathrm{\times }\frac{\mathrm{\&pgr;}}{\mathrm{2}}\mathrm{,}$$

wherein "sign" is a function whose value is +1 when the argoment is positive and -1 when the argoment is negative, while "isneg" is a function whose value is 1 when the argoment is negative and 0 when the argoment is positive.

The above equation allows the position of the winding arm to be univocally calculated without requiring any zero-search procedure, as well as the speed to be calculated by derivative. The programming of control unit UC, in order to automatically perform the above calculations, belongs to the normal knowledge of the person skilled in the art. Therefore, no further description will be given about it.

A preferred embodiment of the invention has been described herein, but of course many changes may be made by a person skilled in the art within the scope of the inventive concept.